In high data rate communications systems, such as selected satellite communications systems, data transmission typically employs high power amplifiers such as traveling wave tube amplifiers (TWTAs) or solid state power amplifiers (SSPAs). Such high speed communications systems typically require a relatively high output power so that the signal being transmitted can travel greater distances before being significantly attenuated. However, such power is limited by several considerations, including the limited energy generation and storage in the satellite vehicle. In these types of communications systems, low frequency digital baseband signals comprising the stream of digital data bits are transmitted after being modulated onto a high frequency carrier wave.
Various modulation schemes exist and distinguish between the digital bits. Examples of digital modulation schemes include amplitude-shift keying (ASK), binary phase-shift keying (BPSK), quadrature-phase shift keying (QPSK), and quadrature amplitude modulation (QAM). Further, the digital baseband signal may be multi level (M-ary) signals requiring multi level modulation methods.
Quadrature modulation schemes provide both amplitude and phase modulation of the carrier because both complex and imaginary representations of the signals are used. In quadrature amplitude modulation schemes, such as QAM, each bit is converted through a bit symbol representing a complex value having an in-phase, real component and a quadrature-phase, imaginary component. Each bit is represented on a graph having an imaginary axis and a real axis to form a constellation pattern representing a group of signals positioned within a circle around the origin of the axes. The distance from the origin represents the amount of power being transmitted. For example, four bits transmitted at a particular time may be represented as 16 symbols. Each symbol of the pattern identifies a complex voltage value having an in-phase component and a quadrature-phase component and represents the complex voltage value for a particular symbol, which is the time during which each symbol is transmitted. The symbols of the constellation pattern are geometrically spread so that they are more equally spaced apart to more readily distinguish the symbols and reduce bit errors. The constellation patterns are processed through the transmitter without being distorted so that the bits are readily distinguishable from each other at the receiver end.
High power amplifiers are desirable in high speed communications applications because they provide high gain over wide bandwidths. However, the input signal to a high power amplifier must be controlled because the high power amplifier exhibits non-linear transfer characteristics. At lower input powers, the output-input power relationship of the high power amplifier is approximately linear. At peak power output, the high power amplifier saturates, and further increases the input power beyond the saturation point actually decrease the output power of the amplifier.
Non-linear amplifiers are inherently more power efficient at creating radio frequency (RF) energy from direct current (DC) energy but create distortions in the process. Such distortions significantly complicate utilizing traditional signal constellations, such as M-ary QAM. Non-linear channels cause the constellation to rotate and expand non-uniformly. Various methods are available to compensate for this expansion and rotation, but such methods are complex and may be difficult to implement.
The non-linearity of the high power amplifier affects the position of the true-invention is particular at predistortion symbols in the constellation pattern by moving them away from the origin. It is known to provide amplifier predistortion techniques in the amplifier when the transmitter is being operated in its non-linear range near peak output power.
Thus, it is desirable to provide an efficient communications system utilizing a peak-power-limited, non-linear channel which compensates for distortion.